This Masonry is for the Bees!

Stone house among trees.
The Stonehaus, High Amana, Iowa.

National Park Service

A sandstone building, known as the Stonehaus, is located in one of Iowa’s National Historic Landmark (NHL) districts, the Amana Colonies. The home was recently repointed and had portions of the lower walls parged. [1] Because of an incompayibility between the hard pointing/parging materials and the soft sandstone, plus the accelerated introduction of moisture to this area, the sandstone is “melting” away, leaving the harder mortar joints intact. This process produces the appearance of a honey comb.

Home and business owners with historic masonry buildings such as the Stonehaus often make this mistake. Wanting to keep their building sound, they undertake what they believe to be responsible preventative maintenance to keep the masonry (brick or stone) from deteriorating further. This is done, unfortunately, without a good knowledge of how a masonry building acts and reacts to various treatments and the weather over time; or how a masonry building “breathes.” Plus, every hardware store in the country carries “quick-fix” remedies of all types for the “Do-it-Yourselfer.” Included are pre-packaged mortar repair “kits.” Unfortunately, these kits contain a high degree of portland cement, which produces a very hard surface. [2] This is fine for modern materials, and modern repair techniques, but is disastrous for older masonry buildings.
Stone with holes in it.
Honey-combing on the Stonehaus, High Amana, Iowa.

National Park Service

Some historic masonry buildings ‐‐particularly historic brick buildings ‐‐are more susceptible to honey‐combing mainly due to their age and types of materials used: both the masonry units and the mortar. In particular, historic bricks were fired much differently than those of today. Modern bricks are fired at much higher temperatures resulting in much harder units. The softer bricks (or stone) are just not compatible with high‐content portland mortars, which produce very hard, and unforgiving, surfaces. [3] In addition, early mortars were much more forgiving, softer, and had a “healing” effect when dampened. Since early bricks were also fired “softer,” the joining of these two materials was ideal. They performed in concert, breathed together, and moved at approximately the same rate.
The new pointing mortar and parging material used on the Stonehaus contained a high degree of portland cement, verified by a physical test of these areas. Two problems contributing to the deterioration of the masonry immediately above the parged surface are apparent: 1) the parging ‐impervious to moisture ‐allowed ground water to “wick” up the masonry and concentrate immediately above the parged area. This action saturated the stone and began a process of breaking apart the chemical bonds holding the stone together, causing it to deteriorate; 2) the portland pointing mortar, which has a lower porosity than the sandstone, is much, much harder than the stone. As a building “moves” due to thermal expansion during the day and evening hours, the different hardness of adjacent materials causes one to “give” while the other remains constant. Another problem with the high portland procontent parging is that it cannot breathe (or allow the masonry behind it to breathe), moisture will be trapped in the stone behind this coating, causing the masonry to deteriorate at a highly accelerated rate. If we removed the portland parging, we would most likely find very poor quality sandstone, or perhaps only sandstone powder.
Brick wall
The action of freeze-thaw has begun to deteriorate the mortar in this wall.

National Park Service

Two additional sources or causes of masonry decay in historic masonry buildings should be noted here. The first is freeze‐thaw action – where moisture enters masonry and in the winter freezes, causing an unwanted expansion of the masonry units, the mortar or both. The second is an action called “rising damp,” caused by moisture wicking up masonry walls through capillary action (the dryer portions of the masonry attract moisture much like a sponge). [4] Often seen in these cases (such as basements) is a “high tide” line – the junction between the rising damp and the dry wall above, often occurring 3‐to-4 feet above basement floor level. The deposition of soluble salts or “efflorescence” can often be seen at this tide line – a whitish, fuzzy layer which can be brushed off. These soluble salts are carried by evaporating moisture moving to the surface of the masonry and deposited there. The problem with masonry in this condition is that moisture has started to break down the chemical bonds which actually hold the masonry together. Delamination, surface chipping, or disintegration can occur. Uncorrected, this action will render individual masonry units or a whole wall of historic masonry inadequate to carry the weight of the wall ‐the end result being collapse.
Brick wall
An example of rising damp. The whitish deposits are soluble salts, referred to as "efflorescence."

National Park Service

Accelerated decay like this is not only unattractive, but in the case of the hidden masonry wall behind the parging, the structural integrity might be compromised without anyone knowing about it in time to prevent collapse.

As with living organisms, historic masonry buildings must be allowed to breathe. When repairs are considered, coating with an impervious material such as high‐content portland cement is never recommended. If pointing is considered, a pointing mortar should be chosen that duplicates the old mortar in strength, composition, color, and texture. [5] Consult a preservation professional for appropriate mortar "recipes." If replacement materials (stone or brick) are required, a similar approach is necessary (choose masonry materials that duplicate the existing in strength, composition, color, and texture). Preservation Briefs No. 2 and 39 are excellent sources of information.
Originally published in "Exceptional Places" Vol. 2, 2007, a newsletter of the Division of Cultural Resources, Midwest Region. Written by Mark Chavez.

[1] Also known as “parget,” a mixture such as plaster or cement used to waterproof outer walls and line chimneys.
[2] “A History of Cement,” The Portland Cement Association, http://www.cement.org/index.asp
[3] For more information please refer to NPS Preservation Brief No. 2, “Repointing Mortar Joints in Historic Masonry Buildings,” Robert C. Mack, FAIA, and John P. Speweik; and NPS Preservation Brief No. 39, “Holding the Line: Controlling Unwanted Moisture in Historic Buildings, Sharon C. Park, AIA. National Park Service, Technical Preservation Services, http://www.nps.gov/history/hps/TPS/briefs/brief02.htm / ... brief39.htm
[4] Capillary action occurs when moisture in saturated porous building materials, such as masonry, wicks up or travels vertically as it evaporates to the surface. In capillary attraction, liquid in the material is attracted to the solid surface of the pore structure causing it to rise vertically; thus, it is often called "rising damp," particularly when found in conjunction with ground moisture.
[5] The Secretary of the Interior’s Standards for the Treatment of Historic Properties, masonry guidelines, http://www.nps.gov/history/hps/tps/standguide/preserve/preserve_masonry.htm

Last updated: July 3, 2018